1. Field of the Invention
The present invention relates to an optical information recording medium, and more particularly to a phase-change type optical information recording medium wherein a phase change, resulting from the application of a laser beam thereto, is produced in a material which forms a recording layer to provide an information recording medium which is capable of recording information, reproducing recorded information, erasing recorded information, rewriting or overwriting information at low linear velocity of disc rotation, and is suitable for application in optical memory related equipment, in particular, rewritable compact disc (CD).
2. Discussion of Background
One commonly known type of optical memory medium capable of writing, reproducing and erasing of information by the application of electromagnetic waves, in particular, a laser beam is the phase-change type of recording medium which utilizes the transition from a crystalline phase to a non-crystalline or amorphous phase or from one crystalline phase to another crystalline phase. In particular, with respect to this type of recording medium, research and development has recently become very active because overwriting using a single beam, which is difficult in the case of a magneto-optical memory, is possible, and the optical system on the drive side for this recording medium has a simpler mechanism than the optical system for a magneto-optical memory.
Chalcogen type alloy materials such as Ge--Te, Ge--Te--Sb--S, Ge--Te--S, Ge--Se--S, Ge--Se--Sb, Ge--As--Se, In--Te, Se--Te, Se--As and the like, as disclosed in U.S. Pat. No. 3,530,441, can be given as representative examples of a material for the phase-change type recording medium. Furthermore, it has been proposed to add to the Ge--Te alloys, materials such as Au as disclosed in Japanese Laid-Open Patent Application 61-219692, Sn and Au in Japanese Laid-Open Patent Application 61-270190, and Pd in Japanese Laid-Open Patent Application 62-19490 with the object of improving the stability and speed of crystallization. A material with a specified composition ratio for Ge--Te--Se--Sb as disclosed in Japanese Laid-Open Patent Application 62-73438 has also been suggested for improving the write/erase repeatability characteristics. However, none of these methods completely satisfies the many characteristics required in a phase-change type optical memory medium in which overwriting is possible.
Furthermore, in Japanese Laid-Open Patent Application 63-251290, there is proposed an optical information recording medium with a recording layer composed of a multi-component (substantially tertiary or more) compound whose crystalline state is of a single phase. This type of recording medium is hereinafter referred to as "optical recording medium". An example of such a single phase multi-component compound is In.sub.3 SbTe.sub.2. In a recording layer containing such a compound, the amount of the compound is 90% or more. By use of such a recording layer, high speed recording and erasure become possible. However, the laser power now available for recording information in the recording layer and erasing the recorded information therefrom is insufficient, so that such a recording layer has the shortcoming of a low erase ratio.
In Japanese Laid-Open Patent Application 1-277338, there is proposed an optical recording medium comprising a recording layer which is composed of an alloy represented by the following formula: EQU (Sb.sub.a Te.sub.1-a).sub.1-y M.sub.y
wherein 0.4.ltoreq.a&lt;0.7, y.ltoreq.0.2, and M represents an element selected from the group consisting of Ag, Al, As, Au, Bi, Cu, Ga, Ge, In, Pb, Pt, Se, Si, Sn and Zn.
The basic composition of this alloy is Sb.sub.2 Te.sub.3. By increasing the amount of Sb excessively, the high speed erasability and repeated use characteristics of the optical recording medium are improved. Furthermore, the addition of M to the basic composition further improves the high speed erasability and erase ratio. However, nothing is mentioned about the erase ratio at overwriting in this Japanese Laid-Open Patent Application. According to tests conducted by the inventors of the present invention, incomplete erasure was observed and the recording sensitivity was insufficient for use in practice.
In a recording layer of an optical recording medium disclosed in Japanese Laid-Open Patent Application 60-177446, an alloy represented by the following formula is employed: EQU (In.sub.1-x Sb.sub.x).sub.1-y M.sub.y
wherein 0.55.ltoreq.x.ltoreq.0.80, 0.ltoreq.y.ltoreq.0.20, and M represents Au, Ag, Cu, Pd, Pt, Al, Si, Ge, Ga, Sn, Te, Se or Bi.
Furthermore, in a recording layer of an optical recording medium disclosed in Japanese Laid-Open Patent Application 63-228433, an alloy represented by the following formula is employed: EQU GeTe--Sb.sub.2 Te.sub.3 --Sb(excessive).
None of these optical recording media, however, exhibits satisfactory recording sensitivity and erase ratio. In particular, the most important problems to be solved are the improvement of write and erase sensitivity, prevention of lowering of erase ratio resulting from failure of erase prior to overwriting, and improvement of the life of both the recorded and unrecorded portions in the recording layer.
In order to solve these problems, the provision of a chemically stable, heat resistant protective layer on both sides of the recording layer has been proposed in Japanese Laid-Open Patent Applications 61-5450 and 63-259855.
Generally the heat resistant protective layer is required to have the properties, for instance, of being transparent to a laser beam, and having a melting point sufficiently higher than the temperatures at the recording and erasing operations, high mechanical strength and high stability to chemicals. Of these properties, stability to chemicals is significantly important when the heat resistant protective layer is used for a phase-change type recording layer, in particular in the case where chalcogen type compounds are employed in the recording layer, since chalcogen elements in the compounds are very active.
Conventionally employed oxide-based dielectric materials generally do not satisfy the above requirements.
The heat resistant protective layer also has a heat radiation or heat transfer function.
When the thermal conductivity of a thermal radiation layer is too small, a rapid cooling effect necessary for making the recording layer amorphous cannot be obtained, while when the thermal conductivity thereof is too large, the applied heat cannot be used effectively, so that the conditions for gradual cooling of the alloy in the recording layer cannot be appropriately set. As a result, the write and erase sensitivity is decreased in both cases. Thus it is necessary to adjust the thermal conductivity of the heat radiation layer in such a manner as to be suitable for the recording layer. However, it is extremely difficult to adjust the thermal conductivity of the heat radiation layer to a wide range so long as the above-mentioned conventional materials are employed in the heat resistant protective layer.
For instance, when the linear velocity of disc rotation for recording by a laser beam is 6 m/sec or less, namely 4 times the standard linear velocity of disc rotation (about 1.2 m/sec to 1.4 m/sec) for a conventional compact disc, the cooling rate on recording, that is, on the absorption of the energy of a laser beam applied, is insufficient, so that appropriate recording marks cannot be formed in the recording disc.